Botulism is a disease caused by a neurotoxin that is considered to be the most potent and lethal known to mankind.

Photo: Courtesy University of Pennsylvania School of Veterinary Medicine

Veterinarians at the University of Pennsylvania School of Veterinary Medicine's (Penn Vet) New Bolton Center have completed important research on botulism in horses, showing that adult horses that maintain the ability to stand on their own have a much better rate of survival in a hospital setting than those that lose the ability to stand.

The study showed that if an adult horse was able to rise on its own throughout the disease, 95% survived and had a full recovery. If the horse lost the ability to stand, however, only 18% survived.

“If the horse maintains the ability to stand, the horse has an excellent prognosis of a full recovery,” said Amy Johnson, DVM, Dipl. ACVIM, assistant professor of large animal medicine and neurology at Penn Vet’s New Bolton Center.

In addition, if the horse was given antitoxin (the only known treatment for botulism) within six hours of admission, it had a much better chance of survival—121 times better—than horses that did not receive antitoxin.

Background on Botulism

Botulism is a disease caused by a neurotoxin that is considered to be the most potent and lethal known to mankind. The bacteria that produce it are everywhere, residing in our soil.

However, these bacteria only produce the neurotoxin under certain conditions, specifically moist, low-oxygen environments. Adult horses are most commonly exposed to the neurotoxin through the ingestion of spoiled forage, such as a large bale of hay that was not harvested or stored properly.

Many species are susceptible, but it is believed that horses are the most susceptible to botulism, showing clinical signs after ingesting a relatively low dose of the toxin.

The disease is characterized by progressive muscle weakness. Initially horses have difficulty swallowing because of pharyngeal and laryngeal (throat) muscle weakness. As more and more muscle groups are affected, the horse has more and more difficulty moving around, and eventually loses the ability to stand. Once it involves the respiratory muscles, the horse usually dies of complications.

Johnson; Helen Aceto, VMD, PhD, associate professor and director of biosecurity at New Bolton Center; and Susan McAdams-Gallagher, a technician in the Penn Vet Botulism Reference Laboratory, recently completed a research study on the effect of botulism on adult horses. They worked toward concrete results that could help veterinarians advise owners on whether to treat their horses for botulism.

“We were trying to identify predictors for non-survival,” Johnson said, “so that I could better counsel owners when they brought patients to me on whether or not it is fair to the horse and fair to the owner to continue treatment.”

The Human Connection

Johnson, who specializes in neurology, was prompted to do the study after learning more about botulism from a person paralyzed by the disease.

“I was shocked when I listened to her description,” Johnson said of the patient. “She described unrelenting physical pain. I didn’t think of this as a painful condition. She was overwhelmingly frustrated because she couldn’t communicate. She was scared all the time.”

“Pain. Frustration. Fear. This really struck me because I realized this must be what horses experience,” Johnson continued.

Johnson surmised that horses must have some degree of psychological stress and physical pain, especially if these large, heavy animals are recumbent, lying on a thin layer of bedding on a concrete floor. They are prey animals that suddenly have lost the ability to escape from real or perceived predators. Owners also suffer psychological stress, and face financial repercussions, as an average hospital bill for a botulism case is thousands of dollars.

“You start to think about the costs of this disease when you are dealing with an individual animal,” Johnson said. “Is it fair and reasonable to continue treatment in cases like this? What is the prognosis? Is recovery possible?

“We know full recovery is possible for botulism; they can regain muscle strength,” Johnson said. “But because of complications from managing these horses, what percentage actually survives?”

In researching the topic, she found no data about survival rates for adult horses with botulism in hospital settings. The current literature only includes outbreak situations with limited treatment efforts and low survival rates. And there is data on foals, but they get a different form of the disease. Foals have an excellent survival rate in a hospital setting, over 90%.

The Study

The research study’s hypothesis was that adult horses treated for botulism at a veterinary hospital would have improved survival in outbreaks, but the survival rate would not be as good as those for people or for foals. The objective was to establish a survival rate for adult horses, and determine indicators that could help predict the likelihood of death.

Botulism is characterized by progressive muscle weakness throughout the body.

Photo: Courtesy University of Pennsylvania School of Veterinary Medicine

The study focused on adult horses with botulism treated at Penn Vet’s New Bolton Center from 1989 to 2013. The study included 92 horses with a median age of five. Of that group, 42 were mares, 13 were stallions, and 37 were geldings.

Key findings included:

Of the 92 horses, 86 were treated.

Of those 86 horses, 37 did not lose the ability to stand on their own throughout the treatment. Of those, 35 horses lived, a 95% survival rate.

Of the 49 that were recumbent, only nine lived (an 18% survival rate).

The survival rate for all treated horses was 48%.

“If they walk off the trailer at the hospital, they already have a much better chance at survival,” Johnson said. “If they are dragged off the trailer, then the chance of survival is much less.”

The study included information provided at admission and clinical findings.

The most common clinical sign was weak tongue, present in nearly every case, followed by weak eyelids, weak tail, and difficulty swallowing in about three quarters of the cases.

The most prevalent treatment was intravenous fluids, since the horses were often dehydrated. The other was antitoxin, which doesn’t reverse clinical signs, but can stop progression of signs, preserving remaining muscular function until recovery occurs. The antitoxin treatment has to be administered early in the course of the disease to be most effective.

Other treatments included tube-feeding, since patients often couldn’t eat or drink an antimicrobial drug to prevent infections.

For the patients that survived, they regained the ability to eat and drink after a median of eight days, and they regained the ability to stand (if they'd lost it) after a median of 13 days. The median hospital stay for survivors was 14 days.

Non-survivors were in the hospital for a median of one day, and two-thirds of those that didn’t survive were euthanized within 24 hours.

Of those that survived, nearly two-thirds had complications, ranging from pneumonia to colic to ulcers on pressure points from lying down. But these complications had no effect on recovery, Johnson said.

“We can tell owners that even if their horses develop these complications, do not lose hope,” she said, “because they are not affecting the chance of survival.”

Owners should contact their veterinarians as soon as possible if they suspect botulism, she said: “Early intervention is critical. Oftentimes we see these horses later than we wish we had."

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